Biosynthesis of gold nanoparticles using papaya seed extract for the functionalization of nanocellulose membranes

Abstract

Papaya seeds are a byproduct with virtually no commercial purposes nowadays, although they are a promising renewable source of many types of chemical compounds, with high contents of antioxidants. These chemicals have the potential to act as reducing agents in the synthesis of metallic nanoparticles as well as having antibacterial and antioxidant activities. Biosynthesized nanoparticles can be used to improve the commercial use of papaya seeds, mostly by doping nanocellulose membranes with these nanoparticles, allowing the production of sustainable and biocompatible nanocomposites with noteworthy properties. Here, we report the preparation of papaya seed extract in ethanol to use as a reducing agent in the biosynthesis of gold nanoparticles (AuNPs) since natural extracts are considered a greener, low-cost, and efficient approach. For comparison, AuNPs synthesize with sodium citrate as a reducing agent were utilized as a positive control. Further, the biosynthesized AuNPs were impregnated in nanofibrillated cellulose (NFC) membranes at concentrations of 0.5%, 1%, and 2% based on a strategy of biomass valorization. Papaya seeds had a 30% yield in ethanol, and a diverse composition of antioxidant chemicals, such as flavonoids and phytosterols. The AuNPs biosynthesized with papaya seed extract showed a surface plasmon resonance band at 545 nm. Due to the variable content of the ethanolic seed extract, the biosynthesized AuNPs had different morphology than the reference, showing distinct shapes and sizes ranging from 15 to 60 nm. In terms of shelf life, both reference and biosynthesized AuNPs presented better stability at low-temperature storage. Doping of NFC membranes revealed that the biosynthesized AuNPs would have a higher nanoparticle concentration in the biosynthesis and stronger adhesion to the NFC than the reference nanoparticles, especially for a 2.0% doping concentration. Also, they presented a homogeneous distribution across the cellulose nanofibrils with several sites impregnated with AuNPs. The results highlight a potentially simple and green route using Brazilian papaya seeds to synthesize AuNPs and impregnate NFC membranes for noteworthy applications based on the bioeconomy and biorefinery concepts.